Direct Oxidation of Methane to Methanol Using Copper Phthalocyanines as Precursors

The direct oxidation of methane to methanol (DOMTM) in the liquid phase was investigated using a copper α-3,5-(di-tert-butyl)phenyl phthalocyanine (CuPc) supported on hierarchical ZSM-5 zeolites prepared by alkaline desilication, with H$_2$O$_2$ as an oxidant (50 ºC, 30 bar CH$_4$, 0.5 M H$_2$O$_2$). Catalysts with varying Cu loadings (0.5 - 1.0 wt.%), framework compositions (SiO$_2$/Al$_2$O$_3$ = 23 and 30), and thermal treatment (intact CuPc vs. calcined) were evaluated. Qualitative kinetic analysis was performed through the quantification of the products over time (0-4 h) by HPLC, $^1$H NMR, and potentiometric titration. X-ray absorption spectroscopy (XAS) established that the CuPc macrocycle remains structurally intact after incorporation into the zeolite (Cu-N distance 1.94 Å, coordination number ~4.4), while calcination leads to complete macrocycle decomposition and formation of isolated Cu$^{2+}$-2Z framework species, as confirmed by XANES, UV-Vis-diffuse reflectance spectroscopy, and H2-TPR. Elemental mapping by HAADF-STEM demonstrated that copper is homogeneously dispersed throughout the zeolite. Calcination significantly improved catalytic performance. ... mehrThe catalyst 1CuPc-ZSM-5-30-DS-Calc (Cu/Al molar ratio = 0.17) achieved a CH$_3$OH productivity of 553 μmol g$_{cat}$$^{-1}$ h$^{-1}$ and 47% selectivity at isoconversion (0.1%), corresponding to a methanol yield of 4.3 mol$_{CH3OH}$ mol$_{Cu}$$^{-1}$ h$^{-1}$, surpassing many recent reported phthalocyanine-based systems for DOMTM. An even higher methanol yield of 8.0 mol$_{CH3OH}$ mol$_{Cu}$$^{-1}$ h$^{-1}$ was obtained over 0.5CuPc-ZSM-5-30-DS-Calc (Cu/Al molar ratio = 0.09). The Brønsted to Lewis acid site (BAS/LAS) ratio and Brønsted acid site density were identified as key descriptors of CH$_3$OH selectivity and productivity. These findings establish that CuPc functions primarily as a precursor to well-dispersed Cu²⁺ active sites, and that copper speciation, zeolite acidity, and mesoporosity jointly govern methanol selectivity in this reaction system.